In some modern network designs, the identity and location of network transmission reception points (TRPs) is not known to the user equipment (UEs) served by the network. A specific example is a UE-centric no-cell (UCNC) system in which multiple TRPs may serve a UE without the UE knowing which TRPs are serving it. This presents a challenge in terms of getting accurate channel measurements.
It is possible for the network to conduct measurements based on sounding reference symbols (SRS) transmitted by the UE. This is sufficient for measuring the uplink channel. If the uplink and downlink channels were reciprocal to a sufficient degree, this could also form the basis of an estimate of the downlink channel. However, in general, the uplink and downlink channels are not reciprocal. For example, the carrier frequency for the uplink and downlink may be different, the number of antennas for uplink vs. downlink transmission may be different, or the background noise and interference may be different.
Channel state information (CSI)-reference symbols (RS) may be transmitted by the network. The CSI-RSs contain pilot signals. The UE can measure these and report CSI data back to the network. In a hypercell, the same CSI-RS may be transmitted by a number of TRPs.